Molecular Models of EDTA and Other Chelating AgentsWilliam F. ColemanDeirdre Bell-Oudry presents a variation on an old theme in her paper on using an indirect EDTA titration for sulfate analysis (1). EDTA and (often loosely) related species are this month's Featured Molecules.EDTA is a hexaprotic acid (H6Y2+) having the pKa values given in the featured paper (1). Figure 1 shows a distribution diagram for the EDTA system (2). At the pH of normal waters, the predominant species have one or both of the nitrogen atoms protonated.Complexation, however, requires that both nitrogens be deprotonated and it is generally assumed that the form that complexes with metal ions is Y4−. Structures of several forms of EDTA are included in the molecule collection (Figure 2). These structures are quite flexible having many conformations that are readily accessible at room temperature.An introduction to EDTA chemistry leads to broader questions of metal ion chelation or sequestration. Related chelating agents included in the molecule collection are EGTA, DCTA, NTA, BAPTA, and DTPA. Molecular dynamics and Hartree-Fock calculations on BAPTA (Figure 2) confirm that many conformations, ranging from those with the phenyl rings parallel to one another, to more elongated forms, are essentially isoenergetic in room temperature aqueous solution (3).Also included in the molecule collection are several crown ethers, an isophore (nonactin), and a cryptand. These not only provide students with a glimpse of the types of molecules being employed for metal ion sequestration but open a wide range of topics of current research in a variety of areas of inorganic, industrial, environmental, and biological chemistry.

Ammonia (GCMP)David M. WhisnantAmmonia fountain: this is a resource in the collection "General Chemistry Multimedia Problems". In an ammonia fountain, a flask is filled with ammonia gas. A tube from the flask extends into a pan of water that contains phenolphthalein. When a rubber bulb full of water is squeezed, the water squirts into the flask. Water from the pan then is pushed into the flask and the indicator changes color. General Chemistry Multimedia Problems ask students questions about experiments they see presented using videos and images. The questions asked apply concepts from different parts of an introductory course, encouraging students to decompartmentalize the material.

Principal Species and pHRobert M. HansonCalculates concentrations of principal species in solutions using JavaScript. You can specify whether "1st-year" methods or mass-charge balance methods are used in the calculations. Solutions can be chosen from the included set or you can design your own.

Acids / Bases |

Titration / Volumetric Analysis |

pH |

Solutions / Solvents

Computer Simulations of Salt SolubilityVictor M. S. Gil, João C. M. PaivaComputer Simulations of Salt Solubility provides an animated, visual interpretation of the different solubilities of related salts based on simple entropy changes associated with dissolution: configurational disorder and thermal disorder. This animation can also help improve students conceptual understanding of chemical equilibrium before any quantitative interpretation of equilibrium constants is attempted.

Computational Chemistry |

Solutions / Solvents |

Thermodynamics |

Equilibrium |

Precipitation / Solubility

Molecular Models of Compounds in Maple SyrupWilliam F. ColemanThis month's issue of J. Chem. Educ. includes articles by David Ball dealing with the chemical composition of honey (1) and maple syrup (2). The JCE Featured Molecules for this month are drawn from those papers. In prior months we have included sucrose, glucose, and fructose (3), and all of the naturally occurring amino acids (4) in the molecule collection. This month we add the molecules identified in Table 4 of ref 2 as probable contributors to the taste of maple syrup. This group of molecules could serve easily as a starting point for a variety of student activities in the area of taste. Students in non-majors courses could be asked to identify structural similarities and differences among the various molecules and could be introduced to functional groups. Students could look for other foods in which some of these molecules are found, and could begin to develop a list of molecules contributing to flavor. In the penultimate paragraph of the maple syrup paper there is a list of substances used as flavoring agents in artificial (maple) syrup. What molecules are in fenugreek and lovage that might be important in flavoring? What are the structures of the other molecules in that paragraph and what, if any, structural features do they have in common with the featured molecules? Students in organic or biochemistry courses could begin to explore the chemistry of taste in more detail. Good starting points for this work are The Chemistry of Taste: Mechanisms, Behaviors, and Mimics by Peter Given and Dulce Paredes (5) and the Chemical and Engineering News Web site (6), which includes a number of articles on this subject.

Descriptive Chemistry |

Solutions / Solvents |

Food Science |

Plant Chemistry

Chemical & Physical ChangeAmerican Chemical SocietyEverything you see and touch has the ability to change. Sometimes substances change to form new substances. This is called a chemical change. Other times substances change but keep the same identity. This is called a physical change. Try these activities to learn more about chemical and physical change.

Introduction of Aquatic Chemistry in General Chemistry Curriculum;Spreadsheet Calculation ApproachesChulsung KimFundamental aquatic chemistry concepts may be introduced in general chemistry classes by computing ionization fractions and buffer intensity of aqueous phase carbonate systems. This Excel spreadsheet may used to build graphic presentations of a titration curve, distribution diagram, and buffer intensity as a function of pH. Accompanying activities are designed to enhance the concepts of acid-base equilibrium through exploring the relationship between pKa/pKb, pH of the solution, ionization fractions, and buffer intensity, and to exercise students graphing skills.

Acids / Bases |

Aqueous Solution Chemistry |

pH |

Titration / Volumetric Analysis |

Water / Water Chemistry |

Equilibrium

Characteristics of MaterialsAmerican Chemical SocietyWhat makes diapers absorbent? Is peanut butter stickier than syrup or jelly? Strong, stretchy, sticky, or sweet—everything around us has special properties which make them unique. See if you can identify and compare the characteristics of materials.